Polymers based on vinylidene fluoride CF2═CH2 (VDF), such as, for example, PVDF (poly(vinylidene fluoride)), are known to offer excellent mechanical stability properties, very great chemical inertness and good resistance to ageing. These qualities are made use of in varied fields of application. Mention will be made, for example, of the manufacture of extruded or injection-moulded components for the chemical engineering or microelectronics industry, use in the form of sealing sheaths for the transportation of gases or of hydrocarbons, the production of films or coatings which make possible protection in the architectural field, and the production of protective components for electrical engineering uses.
Patent U.S. Pat. No. 4,025,709 discloses the polymerization of VDF in the presence of potassium persulphate as initiator, of sodium acetate and of a dispersing additive which can be a sodium perfluorooctanoate (ex. 1–3) or a sulphonate of formula Rf—C2H4—SO3M in which M denotes an alkali metal or ammonium. The proportion of dispersing additive with respect to the PVDF formed is between 550 and 2200 ppm. The PVDF is obtained in the form of a latex which is dried in an oven or which is separated by centrifuging to produce the PVDF. There is no washing of the latex and the PVDF therefore comprises between 550 and 2200 ppm of dispersing additive. With this method of drying, the surface-active additive is not removed or is very partially removed, contrary to the claimed invention.
Patent EP 169 328 (U.S. Pat. No. 4,569,978) discloses the polymerization of VDF in the presence of diisopropyl peroxydicarbonate (IPP) as initiator and there is no sodium acetate. Patent EP 816 397 discloses a similar process to the above, except that the initiator is di-tert-butyl peroxide.
Patent EP 387 938 discloses the polymerization of VDF in the presence of ammonium persulphate as initiator and of ethyl acetate as chain-transfer agent without sodium acetate or dispersing additive.
Patent U.S. Pat. No. 5,955,556 discloses the polymerization of VDF in the presence of an oxidizing-reducing couple but without dispersing additive or sodium acetate. The oxidizing agent can be a persulphate.
The PVDF manufactured according to the processes described above exhibits a thermal stability which is insufficient for applications requiring conversion by extrusion, compression moulding or injection moulding.
The prior art has also disclosed the incorporation of salts in PVDF in order to stabilize it thermally. Patent FR 1 298 572 discloses PVDF comprising barium or strontium salts. Patent U.S. Pat. No. 3,728,303 discloses the polymerization of VDF in the presence of ammonium persulphate as initiator but without dispersing additive. Sodium chloride, potassium chloride, sodium chlorate or potassium chlorate is subsequently added to the PVDF. Patent EP 870 792 discloses the polymerization of VDF in the presence of persulphate as initiator and of a dispersing additive which is ammonium perfluorooctanoate. Potassium nitrate is added, either during the polymerization of the VDF or to the PVDF before it is introduced into an extruder. U.S. Pat. No. 5,929,152 discloses the improvement in the stability of PVDF by the addition of a bismuth salt.
It has now been found that by carrying out the polymerization in aqueous dispersion using a persulphate as initiator and that by adding sodium acetate and optionally a potassium alkylsulphonate, a PVDF which is very stable thermally is obtained, provided that the heat stability is not weakened by undesirable surfactant residues. The polymerization is optionally carried out in the presence of a surface-active additive in order to increase the amount of PVDF produced per polymerization batch but, in this case, the PVDF obtained should comprise a level of residual surface-active additive which is sufficiently low not to affect the heat stability. The content of surface-active additive should be less than 300 ppm by weight with respect to the PVDF to avoid a deterioration in the heat stability. To achieve this low level of surfactant residue, it has been discovered that it is sufficient to coagulate the aqueous PVDF dispersion obtained on conclusion of its polymerization, followed by washing with water. The sodium acetate and optionally a potassium alkylsulphonate are then added to the PVDF dispersion and the PVDF powder is recovered by drying.